Asynchronous transfer mode (ATM) transmission systems which carry voice and other forms of isochronous data requires the use of complex and expensive ATM adaptation layer type 1 (AAL-1) segmentation and reassembly processors. This requires that the two ends of a transmission path should communicate timing information so that they can recover from network-induced delays and delay variance in the transmission path. This requires the use of complex digital phase-locked loops and protocols that have not been fully finalized or proven, or proprietary systems.
In addition, AAL-1 is not compatible with billions of dollars worth of installed telephone networking equipment and cannot be deployed without massive and expensive network changes.
For short transmission distances, such as between a subdivision pedestal and a home, or between a PBX and a desk, these complex and expensive protocols are uneconomical compared to existing wire-line solutions that use non-ATM techniques.